This invention concerns hoists and more particularly servo motor powered hoists with controls enabling a “float” mode operation as described in U.S. patent application publication no 2006/0226106. In a float mode, the hoist justs balances a load supported on a hoist chain. The load magnitude is sensed by a load cell mounted so as to generate electrical signals corresponding to the magnitude of the supported load. If the operator pushes up or down on the load, this force is sensed by the load cell and the hoist controls operate the hoist so as to move the load up or down correspondingly. This allows an operator to maneuver a load quickly and accurately by direct contact with the load itself.
With heavy loads or systems which are relatively deflectable, servo motor operation will sometimes induce oscillation of the load on the chain when stopped after being driven up or down due to deflections in the hoist structure. These oscillations will create dynamic variations in the force sensed by the load cell caused by the up and down oscillations and hoist operation will also directly create similar dynamic effects on the load cell readings. If the operator is attempting to maneuver the load while the hoist is in float mode at a time when these dynamic effects exist, this distortion of the load cell readings will mask the forces manually exerted by the operator and interfere with attempts to maneuver the load in that way.
It is the object of the present invention to assist maneuvering of a load by operation of a powered hoist in a float mode by eliminating the effects caused by motions of the supported load.